L ight A mplification by S timulated E mission of R adiation. Spontaneous emission. Stimulated emission. Lecture VIII. LASER. Energy level diagram. The possible energies which electrons in the atom can have is depicted in an energy level diagram. . T he operation of the Laser.
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(Pumping the Laser)
Light: Coherent, polarized
The stimulating and emitted photons have the same:
example: T=3000 K E2-E1=2.0 eV
Probability of stimulated absorption R1-2
R1-2 = r (n) B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) B2-1 + A2-1
assumption: n1 atoms of energy e 1 and n2 atoms of energy e 2 are in thermal equilibrium at temperature T with the radiation of spectral density r (n):
n1 R1-2 = n2 R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)
B1-2/B2-1 = 1
The probability of spontaneous emission A2-1 /the probability of stimulated emission B2-1r(n ):
Condition for the laser operation
Ifn1 > n2
if n2>> n1 - population inversion
Necessary condition:population inversion
How to realize the population inversion?
The system has to be „pumped”
Measurement disturbes the system
Virtual particles: created due to the UP
Three level laser
The laser operation
- optical pumping - occupation of E3 of a short life time,
10-8s. It is a band, the metastable and ground states are narrow :
- electrons are collected on E2: population inversion
- stimulated emission (one photon emitted spontaneously starts the stimulated radiation )
- Beam of photons moves normally to the mirrors – standing wave.
First laser: Ted Maiman
Hughes Research Labs